Find{BLAS,LAPACK}: Avoid clobbering results when no vendor is requested

Logic added by commit 4c74c86f (FindBLAS/LAPACK: Add support for the
Fujitsu SSL2 library, 2021-01-27, v3.21.0-rc1~402^2~1) accidentally
expressed a boolean condition without proper grouping.  The pattern was
then copied by commit 2c9e623e (Find{BLAS,LAPACK}: Add support for the
NVHPC LAPACK library, 2021-05-05, v3.21.0-rc1~192^2).  The resulting
logic incorrectly tries Fujitsu and NVHPC vendors even after results are
found from another vendor, and then erases those.  Fix the grouping.

Fixes: #22403